When an electronic component is mounted by soldering on the surface of a metal layer formed on a surface of an insulating substrate, excessive spreading of solder on the surface of the metal layer needs to be limited. The reliability of soldering is increased by limiting spreading of solder and appropriately confining the solder within a predetermined area. For example, a known technique limits spreading of solder out of a soldering region by forming the entire soldering region as a solder pool that is lower than the other portion (see Patent Document 1).